The present invention relates generally to the production of potato chips and other food products prepared by deep-fat frying, and is particularly concerned with a continuous frying method and apparatus for making potato chips which are similar in taste and texture to those produced by the slow-cooked batch or kettle process.
Large scale commercial production of potato chips is usually carried out in a continuous frying apparatus at temperatures between 300.degree. and 360.degree. F. If frying is carried out below about 290.degree. F., a distinctly different chip results. The chip produced by the lower temperature process is harder and crunchier, and its surface takes on a more bubbly appearance. In addition, the flavor is somewhat blander than that of a regular potato chip, and there is a more pronounced oiliness to the mouth feel. These characteristics are deemed very desirable by many producers and consumers of potato chips. Since the original method of making this type of chip was by the batch, in relatively small vessels of oil heated directly over a flame, the chips were termed "kettle style". Unfortunately, the batch process is very labor intensive and can produce only small quantities of finished product, typically between 50 and 125 pounds per hour per kettle. By contrast, large scale potato chip producers typically employ continuous fryers capable of producing on the order of 1,000 to 5,000 pounds per hour of finished product. The kettle or batch process is therefore uneconomical for large scale manufacturers.
For a number of reasons, it has not been possible to obtain the desirable qualities of kettle style potato chips in chips produced by conventional continuous fryers. In a typical continuous fryer, raw potato slices are introduced at one end of an elongated vessel or trough containing heated frying oil. While immersed in the oil, the potato slices are carried toward the opposite end of the vessel by mechanical conveying devices or more commonly by the velocity of the oil itself. Frying takes place as the water in the potato slices is driven out and replaced with oil, and the completely fried chips are removed when they reach the end of the vessel. In order to maximize the production rate and reduce sticking between the potato slices, high temperatures are utilized and the residence time of the slices in the frying oil can be held to a minimum. These conditions result in a chip with a taste and texture different from that of kettle style chips. Continuous fryers are also characterized by a temperature gradient in the oil bath which decreases continually from a maximum value at the entrance end of the fryer, where the oil is typically admitted after being heated by a heat exchanger or other type of heat source, and the exit end where the oil is withdrawn for reheating and recirculation. By contrast, the time-temperature profile in the kettle or batch process is more complex, initially decreasing as the relatively large amount of water in the raw potato slices absorbs heat from the oil and then increasing after most of the water has been vaporized and driven out. This is another factor which accounts for the differences between potato chips produced by the kettle process and those produced by continuous fryers.
When potato chips are made by the kettle or batch process, there is a tendency by producers to employ the maximum possible density of potato slices in the cooking oil in order to increase the output of what is inherently a very slow process. This increased density results in a high compaction of the potato slices during frying, with the consequence that many of the resulting chips are folded over or crumpled. This folding or crumpling provides a special crunchiness that many consumers find desirable. A second, less desirable result of the increased compaction is that some of the potato slices adhere to each other during the frying process, producing multiple connected potato chips. This phenomenon is undesirable not only because it detracts from the appearance of the final product, but also because the agglomeration of potato slices in the frying oil can lead to incomplete cooking.
Although it would be desirable to produce folded or crumpled potato chips using a continuous frying process, this has proved to be difficult since continuous fryers do not ordinarily achieve the necessary degree of compaction of chips in the frying oil. Although compaction can theoretically be increased by adding a greater quantity of potato slices to the frying oil per unit time, this is effective only to a limited extent. Beyond a certain point, an increase in the density of the potato slices will result in clustering and incomplete cooking of the slices unless sufficient agitation is employed to counteract the effects. Conventional continuous fryers are usually not capable of providing adequate agitation when the potato slices are compacted to the degree necessary to provide folded or compacted chips.